The invention relates to a device for optical distance measurement in geodetic and industrial surveying, according to the features in the preamble of claim 1.
Optical distance measuring devices have wide-ranging applications in geodetic and industrial surveying. Together with an angle measurement, they help to determine the three-dimensional coordinates of target points or target objects in space. The target points to be surveyed are marked by means of reflection-supporting aids. Other target points or target objects are directly sighted as such. From the point of view of measurement, a distinction is made between cooperative and noncooperative target objects. Cooperative target objects are self-luminescent or reflecting target marks, such as spherical reflectors, reflecting foils or triple prisms. Noncooperative target objects are natural, rough surfaces, such as, for example, those of buildings or rocks.
The target objects are sighted by means of a theodolite, which contains a distance-measuring device mounted on the theodolite telescope, or by means of a tacheometer, i.e. a theodolite having an integrated distance-measuring device. The distance measurement is carried out by the phase or transit time measuring principle with intensity-modulated or pulsed radiation. The three-dimensional coordinates of the target objects are determined relative to a specified coordinate system by the angle and distance measurement. Consequently, the coordinates of individual target points can be determined in geodetics, in building surveying or in industrial surveying. Alternatively, surfaces, for example of aircraft, provided with cooperative target marks can be surveyed (W. Huep, O. Katowski: Theodolitsysteme fxc3xar Industrielle und geodxc3xa4tische Messungen [Theodolite systems for industrial and geodetic measurements], in: Technische Rundschau No. 39, 1988, pages 14-18).
On the other hand, it is also possible to pinpoint the coordinates specified on a map or on a building plan by means of a tacheometer and a surveyor""s staff equipped with reflectors. This is usual in the building industry or for locating in road construction. Tacheometers are also used for controlling advancing machines in road construction, tunnel construction and mining.
Conventional electronic theodolites measuring to target marks utilize distance-measuring devices which are integrated or mounted on the theodolite telescope. Virtually all integrated or mounted distance-measuring devices have a biaxial optical system for transmitted and received beams. Furthermore, EP 0 313 518 B1 discloses a distance-measuring device in a coaxial optical embodiment which has an He-Ne laser as a light source and can measure to reflection foils and to natural objects. However, this is a mounted distance-measuring device which as such has a parallax with respect to the theodolite sighting axis and in which the location of the measuring spot and the location sighted via the theodolite are not identical.
The biaxial integrated or mounted distance-measuring devices have a separate, in general laterally offset transmitted and received beam path. This takes into account the lateral offset of the light beam on reflection by retroreflecting target marks (e.g. triple prisms) which reflect an incident light beam parallel and with lateral displacement. In the case of the integrated distance-measuring device, one half of the theodolite telescope lens is used for the transmitted beam and the other half of the telescope lens for receiving the reflected beam. On the other hand, a mounted distance-measuring device has both a completely separate optical axis for the transmitting and receiving optical system and a parallax with respect to the sighting axis of the theodolite telescope. This means that the target point to which the sighting axis of the theodolite telescope is pointed and the location of the measuring spot of the mounted distance-measuring device on the target object are not identical. This is disadvantageous for point measurements. Because of this inter alia, mounted distance-measuring devices are gradually being replaced by integrated distance-measuring devices.
Biaxial distance-measuring devices capable of measuring the distance both to reflector targets and to noncooperative target objects having a naturally rough surface are furthermore known. For example, such devices are used for surveying poorly accessible surfaces, such as in plant construction (cooling towers of nuclear power stations), in bridges, dams, in quarries or in shipbuilding. Further applications are in the profile measurement of tunnels, shafts and roads and in the surveying of building facades. The range is a few hundred meters in the measurement to such noncooperative targets. The biaxiality of these distance-measuring devices gives rise to a parallax which results in an offset of the center of gravity of the image spot. This effect is so pronounced, particularly at close range, that a measurement is not possible without additional technical measures. For example, ancillary lenses are therefore mounted on the transmitting and receiving optical system in the measurement of short distances, as is the case with the distance-measuring device WILD DIOR 3002S from Leica. This implies a certain handling effort. In another technical solution, the parallax is compensated by rotating a rhomboid prism, mounted on ball bearings, as a function of the displacement of the focal lens of the theodolite telescope (biaxial tacheometer Rec Elta RL from Zeiss with two lenses for the transmitting and receiving optical system). With the precisely moved optical and mechanical components, such a compensation of the parallax means a high degree of technical complexity and moreover leads to a large and heavy surveying instrument.
The biaxial distance-measuring devices mentioned so far operate with radiation sources which emit infrared light with large beam spread angles. The light beam diameters are as much as 15-20 cm at a distance of 100 meters. In the case of distance measurements to reflectors, it is true on the one hand that large light beam diameters are advantageous for locating the reflectors. On the other hand, a large light beam diameter for measuring to noncooperative targets leads to greatly reduced local resolution since . the distance value intensity-weighted according to the local reflection properties is measured over the irradiated area. In the case of inclined or structured object surfaces, this does not result in a true distance to the target point of the distance-measuring device. Thus, for example, protuberances present on the object surface and having a small diameter, pipes and cables on facades or in inner rooms of buildings or the structures of window reveals are not measurable owing to the large cross-sectional area of the light beam. Even in the case of short distances of a few meters, the light beam diameter is already several centimeters. Consequently, even steps in surfaces are easily covered by the large measuring spot, resulting in an erroneous distance measurement.
In the case of inclined surfaces on which the measuring light beam is not perpendicularly incident, inhomogeneities of the object surface within the measuring spot can give rise to locally different degrees of reflection. Such inhomogeneities are formed, for example, by soiling, by different surface coatings, moisture or roughness of surfaces. The locally different reflections within the measuring spot result in an unequal weighting in the distance measurement, so that it is not the actual distance to the point of intersection of the sighting axis of the surface to be surveyed that is measured.
Finally, with the use of infrared radiation for the measurement, the actually measured object point on a surface is not detectable. The object point is sighted only indirectly either by means of the telescope optical system of the tacheometer or by the use of the visible radiation of a laser pointer aligned with the sighting axis of the distance-measuring device.
Hand-held distance-measuring devices which operate with a visible measuring beam for noncooperative targets at short distances are furthermore known. DE 40 02 356 C1 describes a distance-measuring device having separate transmitting and receiving beam paths for the distance range of 2-10 m. It contains two laser diodes which are switchable in an electronically complementary manner, one of which sends its light wave train along the measuring distance to the target and the other sends its light wave train along a reference distance inside the device for calibration purposes. The two light wave trains are received alternately by the same photodetector. The journal xe2x80x9cIndustriexe2x80x9d, 11/92, page 6-8, describes a distance-measuring device DME 2000 from Sick GmbH, which operates with two semiconductor laser diodes. The required transmitted light is generated by a laser diode with an optical collimator system, and the second laser diode delivers the necessary reference signal directly to the receiver. The transmitted and received beams are coaxial with one another. The measured distance to natural surfaces is limited to 4 meters. DE 43 16 348 A1 discloses a, device for distance measurement which has a separate transmitted beam path and received beam path and by means of which it is possible to measure to naturally rough surfaces up to a distance of 30 meters using visible radiation.
It is the object of the invention to provide a device for distance measurement for geodetic or industrial surveying purposes or for the surveying purposes in the building industry, by means of which device it is possible to measure both to cooperative and to noncooperative target objects, which device has a high local resolution also in the case of noncooperative target objects so that the distance to small structures on naturally reflecting surfaces can be measured, by means of which device large and small distances can be measured with geodetic accuracy and by means of which device all types of target objects in any distance range can be visually sighted easily and without effort.
This object is achieved, according to the invention, by the features stated in the defining clause of claim 1. Advantageous embodiments and further developments of the invention are evident from the subclaims.
In the distance-measuring device according to the invention, one or two optical radiation sources which generate two radiation beams are integrated in the transmitter unit. The radiation beams differ in their beam divergence, the radiation of one radiation beam being in the visible wavelength range and being diffraction-limited. The other radiation beam diverges. A common lens is used, both for the radiation directed at the target object and for receiving the radiation reflected or scattered by the target object. In addition, selection means for distinguishing between the two types of radiation or radiation beams are used. In general, this results in a number of advantages in surveying in geodetics, in the building industry and in industrial surveying. The advantages are the adaptation to different types of target objects, the very high accuracy of measurement and reliability of measurement for the total distance measuring range to be covered and easy and fast handling and measuring sequences.
The radiation types used in the distance-measuring device and the associated divergences of the radiation beams are optimally adapted for various types of target objects and distances. In the case of measurement tasks where mm or sub-mm accuracy of measurement is required, the target points are marked with reflectors or reflecting foils. Radiation having a large beam divergence is suitable for this purpose since, where a transit time measuring method is used, inhomogeneities having the same transit time are smoothed by large beam divergences.
Furthermore, large distances are measured with the aid of reflectors. Here too, the large beam divergence is advantageous since, owing to the large beam cross-section at the reflector location, the reflector is easily detected. Infrared radiation is preferably used, so that operators (persons holding the surveyor""s staff) or passers by are not dazzled. This applies in particular in the case of measurements to mobile cooperative targets in the near distance range, where the luminous intensity of the measuring light beam is high.
The approximate sighting of the target object is effected by visual estimation or by simple aids, such as backsight and frontsight, or a diopter mounted on the distance-measuring device. When the distance-measuring device is installed in a theodolite, the theodolite telescope can of course be used for sighting in the case of very large distances.
For the measurement to target objects having naturally rough surfaces, the diffraction-limited visible radiation is used. The diffraction-limited radiation beam has the smallest possible beam divergence and hence a very small beam diameter. Thus, a measuring spot having only a small diameter is produced on the target object. Consequently, even small structures on a surface can be surveyed. It is this that makes it possible at all to survey, for example, thin pipes and electrical cables on facade surfaces or inner rooms or the three-dimensional structure of surfaces themselves. In addition, the measuring spot of the measuring radiation is visible on the target object. The structures are thus sighted with the measuring radiation itself and the size of the measuring spot is detectable. This results in a greater advantage in terms of handling and speed. Moreover, this also results in a high degree of reliability of measurement. The true distance values are determined since sighting beam and measuring beam are identical. The separately arranged laser pointer as a sighting aid or an otherwise necessary view through a telescope optical system for sighting is dispensed with.
Furthermore, when a laser is used as a diffraction-limited radiation source in the visible wavelength range, the laser safety regulations permit higher transmission powers. This circumstance favors the range to noncooperative target objects which, in comparison with the cooperative target objects, have a degree of reflection which is lower by several orders of magnitude.
In addition to the use of diffraction-limited visible radiation for noncooperative target objects in the range of relatively short distances, said radiation can in specific cases also be used for cooperative target points at particularly long distances of many km, owing to the higher radiant power. However, with the small beam diameter, it is difficult to strike the reflector target.
The divergent radiation beam is used for the routine measuring range up to a few km to reflector targets. However, this can also be used for noncooperative target objects where it is not matter of surveying very small structures.
To adapt the measurement to a very wide range of target objects and distances, selection means are provided for the different types of radiation, and radiation beams. The selection means may be various optical filters and/or switching devices. For example, switching between the radiation beams can be effected by the user at the press of a button, so that the user can consciously decide in favor of a specific radiation beam for his measuring problem. However, the switching can also be effected automatically by the distance-measuring device itself, for example as a function of the measurement of the received light intensities or of the measured distance. It is of course also possible, independently of measuring criteria, to switch back and forth constantly between the radiation beams with a certain clock frequency. However, simultaneous and continuous radiation emission of the two radiation beams and corresponding continuous illumination of the target object are also possible, the selection according to the radiation beams being effected only in the received beam path.
The selection of the radiation can therefore be realized in various ways with the aid of different selection means. In the case of the simultaneous emission of the two radiation beams, filters can be mounted as selection means in front of the detector in the received beam path. The filters allow through only one or only the other radiation. For example, a manually operable or motor-driven filter wheel comprising corresponding filters can be used for this purpose. The respective radiation beam is received by a single detector. Alternatively, the received beam path can be divided with the aid of a selective beam divider into two beam paths for one radiation beam each, so that two detectors can simultaneously measure the respective received radiation.
On the other hand, the selection of the radiation beams can be effected in the transmitter unit itself. Thus, electrooptical devices, mechanical devices, filters or a controllable frequency doubler or Q-switcher can be introduced into the beam paths, directly before one radiation source or before both radiation sources and each alternately block or suppress one radiation beam and release the other as measuring radiation. It is of course also possible to switch each of the radiation sources themselves on and off so that only one radiation beam is ever emitted. In these cases, filters are not in principle required in the received beam path. Additional filters are of course useful, however, for eliminating stray light and for achieving a high signal/noise ratio.
The radiation beams are passed alternatively or simultaneously through the lens in the direction of the target object. In principle, all parts of the lens are suitable for this passage. It is therefore also possible to use a plurality of different parts of the lens aperture for the emitted radiation. The reflected radiation can in turn be received by another part of the lens.
Preferably, both radiation beams are passed with the center of gravity of the beam in the center of the lens, i.e. coaxial with the optical axis of the lens. This has the advantage that the receiver axis and simultaneously the optical axis of the lens, often serving as sighting axis, together with the two transmitter axes of the emitted radiation beams, is directed at the target object. The target object is sighted exactly at the same point at which the distance too is measured. Consequently, a very accurate distance measurement is achieved. The coaxiality of the emitted radiation does not necessarily mean that the beam cross-section must be rotationally symmetrical with the axis of the lens. A beam cross-section shaped asymmetrically with respect to the optical axis is also possible. The center of gravity of the area of the beam cross-section should preferably coincide with the optical axis of the lens
The same lens is moreover designed for receiving the reflected radiation. Consequently, the distance-measuring device is at the same time also parallax-free, and it is therefore not necessary to take any additional measures for an otherwise necessary compensation of the parallax. Of course, the lens and associated further optical elements must have corresponding qualities with respect to the transmission of the radiations.
If the distance-measuring device is installed in a theodolite, the angle coordinates of the target object can also be determined in addition to the distance. The theodolite measures the horizontal and vertical angle of the optical axis of the theodolite telescope, directed at the target object, in a predetermined coordinate system. The distance-measuring device can in principle be installed as a separate measuring instrument in the theodolite or mounted on the theodolite telescope. However, additional advantages are obtained if distance-measuring device and theodolite are amalgamated to give one device having only a single lens. The lens then serves both for the distance-measuring device and theodolite telescope. As a result, only a single sighting axis exists. The distance measurement with transmitted and received beam path coaxial with this axis as well as the angle measurement relate to this sighting axis. A very high degree of accuracy in the determination of three-dimensional coordinates of the target object is thus achieved. The three-dimensional coordinates of various types of target objects in various distance ranges are thus measured with high resolution, quickly and reliably.